The present invention relates to a system for controlling an idle speed of an internal combustion engine in accordance with a change in load due to an air conditioner or other auxiliary (accessory devices or units).
An auxiliary device such as power steering pump, air conditioner or alternator of a vehicle make the idling operation of an internal combustion engine of the vehicle unstable. In order to prevent the idle speed (or rpm) of the engine from being made unstable by the load of such auxiliary devices, there is proposed an idle control system which controls the idle air amount so as to ensure a prescribed idle speed in accordance with loads of the auxiliary devices by controlling an idle control valve for regulating the air flow bypassing the throttle valve.
When the air conditioner is switched on during idling, for example, this conventional idle speed control system performs such a feedback control as to reduce a deviation of a sensed actual idle speed from a desired idle speed which is predetermined as a target while the air conditioner is on. An idle speed control system of this type is shown in a Japanese Patent Provisional Publication No. 61-237860.
In the idle speed control system of such a conventional type, a correction quantity of an idle air amount (air flow rate) corresponding to a load increase due to the air conditioner or other auxiliary devices is preliminarily determined. When the load is imposed, the control system performs the feedback control by adding the predetermined correction quantity so as to maintain a stable idling performance in spite of the change of the load.
This conventional system, however, cannot avoid suffering a temporary decrease of the idle speed due to a delay between the detection of increase of the load, and a responsive action of the control system. Recovery from the temporary decrease takes a significant time, during which the engine tends to produce undesirable vibrations. If the correction quantity is set at a high level in order to prevent this temporary decrease of the idle speed, then this control system would suffer an excessive increase of the idle speed by overshoot, and an undesired fall of the idle speed caused by undershoot when the load is removed.